The present invention relates to electro-optical modulators.
More particularly, the invention proposes electro-optical modulators that are fast, i.e. that make it possible to obtain modulation frequencies of the order of 1 MHz or higher.
This type of modulator is, in particular, used advantageously in the field of optical telecommunications to transform an electrical signal into a modulated optical signal.
Such modulation is obtained either by modulating the transmitter laser internally, or else by performing modulation externally. External modulation may be performed by discrete components or by integrated optical components which use various physical effects such as, for example, acousto-optical effects, electro-absorption effects, or electro-optical effects. Among the latter effects, the most commonly used effect is the transverse electric Pockels effect which makes it possible to modulate the refractive index of the propagation medium in proportion to the electric field via an electro-optical coefficient. The electro-optical coefficient depends on the non-linear optical properties of the material used. Such a material is said to be "active" because its refractive index varies significantly with the electric field to which it is subjected.
The Pockels effect may be obtained with various types of material, and in particular with light guides made using:
suitable semiconductors;
inorganic or organic crystals that are not centrally symmetrical;
electro-optical polymers; or
passive materials associated with electro-optical materials.
The invention relates to Pockels-effect electro-optical modulators whose light guides are made using passive materials associated with electro-optical materials.
The advantage of this type of structure is that it can be connected easily to other passive components on an optical integrated circuit platform.
For examples of Pockels-effect modulator structures using passive materials, reference may advantageously be made to the following publications:
M. Wilkinson et al., Electronics Letters, vol. 27 (11), p. 979, (1991);
R. A. Hill et al., Proceedings OFC'96, Technical Digest, WL1, P. 166, (1996); and
G. Fawcett et al., Electronics Letters, vol. 28 (11), p. 985, (1992).
Usually, such modulator structures are constituted by passive waveguides which are made of silica or of glasses treated by ion exchange and on which respective polymer films are deposited. Such a structure takes advantage of the evanescent wave coupling that takes place between the core and the polymer at well-defined wavelengths, since the polymer has a refractive index that is considerably higher than the refractive index of the core.
Unfortunately, such a structure suffers from large losses at those wavelengths, and modulation takes place with power levels that are low.